This NCRR shared instrumentation proposal seeks funding for the purchase of an Applied Biosystems 4000 Q-TRAP HPLC-MS triple quadrupole linear ion-trap mass spectrometer with nano-electrospray ionization source. This integrated platform will include a Dionex on-line nano-capillary HPLC system with autosampler and switching device for sample enrichment. The Q-TRAP mass spectrometer will be equipped with a Turbo V Source, a TurbolonSpray probe, a NanoSpray Source and Interface. A full complement of instrument software required for data acquisition and data processing is also requested to support the research aims described in this proposal. In brief, we propose to operate this instrument almost exclusively (>95% use) for on-line nano HPLC-MS/MS for the separation and analysis of complex protein/peptide mixtures. These analyses will emphasize the high sensitivity and special scan features of the 4000 Q TRAP, including multiple reaction monitoring, and precursor ion and neutral loss scanning functions. These unique capabilities will be directed at the identification and quantitation of a wide array of biologically important enzymatic and non-enzymatic posttranslational modifications, including protein phosphorylation, oxidative damage and nitrosylation. The primary, but not exclusive focus, will be on supporting a wide array of NIH-funded projects at the Buck Institute that seek to understand the molecular basis for aging and age-related diseases as they correlate to changes in biologically relevant posttranslational modifications.